All in the Timing

Over the years, astronomers have gained new perspectives on the universe by exploring
sections of the electromagnetic spectrum invisible to human eyes. More
subtly, they have also learned to broaden their perspective on time,
looking for events that happen so quickly that we might never notice them.
The National Aeronautics and Space Administration's orbiting Rossi X-ray Timing
Explorer (RXTE) has a clever talent for both kinds of insight (see sidebar). It
focuses on the energetic X-rays that originate in violent processes
occurring around hyperdense objects such as neutron stars and black holes. And unlike previous x-ray
observatories, RXTE can observe lightning-fast flickerings--thousands of
times per second, in some cases--that reveal unprecedented details of
their underlying phenomena.

The x-ray
sky looks very different than the familiar one. When seen though RXTE'
s eyes, the sky flares with radiation from a class of variable stars known
as x-ray binaries. In these misfit duos, one member has evolved
into a neutron star, a dense stellar corpse just 20 kilometers in diameter-
-or into an even smaller yet more massive black hole. In either case, the
collapsed star's powerful gravity snatches material from its partner, a
more sedate star like the sun. Gas spiraling inward grows fiercely hot,
emitting the observed X rays. In some instances, the gas collects on the
star's surface until it reaches a critical mass, touching off a tremendous
thermonuclear detonation.

Or so the theory goes--nobody understands the exact details of what
happens around a neutron star. But using RXTE, people like Tod E. Strohmayer of the NASA Goddard Space Flight Center
are starting to find out. In a recent paper in the September 20, 1996 Astrophysical Journal Letters, Strohmayer and his colleagues report that the emissions from one X-
ray binary fluctuates an astounding 1,100 times per second
, much faster than any variation ever before observed. "The first thing
you say when you see something like that is, this can't be!" he exclaims.
M. Coleman Miller of the University
of Chicago thinks the X-ray stuttering is a kind of beat pattern that
results from the overlapping periods of the neutron star's rotation and
the cyclic orbiting of hot gas about to crash onto the star's surface. For
the first time, it seems, we can actually watch a neutron star feeding.

Related RXTE studies may finally settle the mystery concerning the origin
of a group of astronomical speedsters called millisecond pulsars. About 15
years ago, radio astronomers discovered that some pulsars (spinning neutron
stars that emit pulses of radiation) have rotation periods of just a few
thousandths of a second, a billion times more rapid than the sun's
rotation. Startled theorists proposed that these pulsars might be born in
x-ray binaries, where the disk of gas crashing onto the neutron star could
give it a powerful kick of angular momentum.

RXTE observations of three star systems containing pulsars that emit
brilliant bursts of X rays bolster the speculation.
Those bursts are thought to result from the episodic nuclear explosions
taking place on the surfaces of the neutron stars in these systems; the
resulting hot spots act as beacons that, for a short time, allow
astronomers to observe directly each neutron star's rotation, even through
the overlying clouds of infalling gas. Strohmayer reports that the
oscillation period (and so presumably the rotation period of the
underlying star) during bursts is just 1/600th of a second--much shorter
than the spin rate of known newborn pulsars and a strong sign that these
stars are in fact in fact being sped up until centrifugal effects nearly
tear them to pieces.

But the process is far from cut and dried. Jean H. Swank of Goddard, the project
scientist for RXTE, notes that neutron stars in some other x-ray binaries
appear to slow down at times. This paradoxical phenomenon may be caused by
magnetic interactions between the star and the surrounding accretion disk,
though slipping and sliding between the layers of nuclear material that
make up the star may also play a role.

These findings are only the beginning. Swank hopes that RXTE will
eventually detect x-ray variations caused by oscillations of the surfaces
of neutron stars. Such observations would permit astronomers to perform a
kind of remote seismology, tracing out the stars' internal structure. That
information would offer powerful insight into the nature and behavior of
neutrons and quarks, the building blocks of the centers of atoms.

Swank also notes that RXTE is looking far beyond our galaxy to study the
emissions from quasars, object hardly larger than our solar system that outshine
entire galaxies. Most astrophysicists believe that quasars are in some
ways like giant X-ray binaries, except that the central object is a black
hole having as much as a billion times the mass of the sun--a beast
capable of swallowing entire stars.

Herein lies a beautiful irony. The rays we see from quasars have been
traveling earthward for hundreds of millions of years or more--and yet
their deepest secrets might be resolved, literally, in the blink of an eye.